Calling Rudy Giuliani unqualified to lead, the city’s firefighter union is vowing to try to torpedo the former mayor’s bid for the presidency.

Uniformed Firefighters Association president Steve Cassidy told The Post that if the Republican front-runner starts to get “real traction, we will go after him.”

The UFA “will never be with Rudy Giuliani,” he proclaimed. “We will make it known that he is not qualified to lead.”

Cassidy’s blunt assessments stemmed from the “poor preparations” Giuliani made to protect the city and first-responders in the wake of the 1993 World Trade Center bombing that killed six.

“For someone running for the highest office in the country claiming to be a leader on terrorism, Giuliani’s track record stinks,” Cassidy declared.

The 51-year-old labor leader even took aim at the iconic image of a dust-covered Giuliani sprinting away from the collapsing Twin Towers.

“If [Giuliani] had a command center that worked, he wouldn’t be in those photographs looking like he was doing something heroic,” Cassidy said, referring to the mayor’s decision in 1999 to site the city’s emergency-management bunker in the World Trade Center.

“All he was doing was wandering around the city, and he wasn’t able to make sure firefighters and police could communicate.

“Serious mistakes, crucial mistakes were made,” Cassidy fumed, pointing to the faulty radios that had failed in 1993 and failed again in 2001, preventing hundreds of firefighters from hearing orders to evacuate the north tower.

“Rudy Giuliani did not have New York City prepared for a second terror attack, and prior to September 11th his poll numbers reflected that he couldn’t get elected to a significant position,” the UFA head raged. “What did he do in the weeks and months after 9/11 except claim he was the guiding light?

“Cops, firefighters and workers cleared that site in record time. We’d do it again, but we certainly want to have someone at the helm of this country who is a proven leader.”

The UFA’s bitter enmity is partially rooted in a Giuliani decision made a few days after 9/11 that limited firefighter access to the Ground Zero site. Firefighters, enmeshed in the grisly work of reclaiming lost colleagues’ remains, erupted with a fury that led to fisticuffs with cops enforcing the mayor’s order. The UFA has nursed anger ever since.

But Cassidy said his current criticisms are unrelated to union business. He also lashed out at Giuliani for campaigning on “revisionist history,” and excoriated the candidate for “not lifting one finger” to help sick Ground Zero workers who had to push Albany legislators to pass 9/11 health-care bills after hundreds of first responders fell ill.

Not all firefighters share the UFA’s visceral dislike of the former mayor. Several firefighter groups around the country have endorsed Giuliani and are campaigning for him. Some of the city’s Bravest are also Giuliani boosters.

But Cassidy, who solidified his conservative bona fides with a 2004 endorsement of President Bush, has clout.

Rudy’s 2008 rival John McCain has already come courting, requesting a private sitdown with Cassidy over breakfast the last time he was in town. The UFA also enjoys a solid working relationship with Democratic candidate Hillary Rodham Clinton, who got the union’s Senate endorsement in 2006 and is considered a strong contender for its 2008 backing.

Cassidy declined to comment on which candidate he preferred. The UFA head is an aggressive campaigner once he makes his choice. In 2004, he lent himself to a four-day blitz through the swing state of Ohio alongside Bush, and sent hundreds of firefighters to Republican rallies to stump for the incumbent.

The UFA, with nearly 9,000 active members and more than 22,000 retirees, has also organized impressive voter-registration, leafleting and phone-bank campaigns for local candidates.

A formal endorsement is still months away, but sources said the union is willing to mount an anti-Giuliani speaking tour among other firefighter groups around the country in the meantime.

“Who we eventually support will be decided by a vote,” Cassidy said. “But I can tell you this, with 100 percent of my board behind me and the majority of my members: It will not be Rudy Giuliani.”